Although mechanical treatment of wood fibres with refiners has been a commercial reality since 1960, the mechanisms involved in refiner pulping are still not thoroughly understood. Refining is a critical step in the pulping process and refining energy for this pulp will typically account for close to a third of the total energy costs associated with newsprint production. As a result, the incentive for optimizing the energy efficiency and pulp quality produced by this part of the process is quite significant.
Developments in the field of refining theory during the early 1980's suggested that fibre development in the refiner is governed not only by the total amount of energy used, but also by the manner in which this energy is applied. In PIRA Int. Conf. New Technologies in Refining (Birmingham, England), 1986, Proc. (vol. 2), Session 4, Paper 11, Danforth introduced the concept of impact intensity, showing that the number of impacts received by the fibres during refining is an important factor in the development of pulp quality. The theory took into account the effects of consistency or solid content, rotational speed, as well as the effect of refiner geometry or design. Recent study of high consistency chip refining by Miles et at. in Journal of Pulp and Paper Science, 1990, 16(2), J63-J71; and Paperi ja Puu, 1991, 73(9), 852-857, has produced a set of equations describing the consistency and pulp velocity profiles in the refiner. These equations make it possible to calculate the residence time of the pulp in the refiner and hence the number of bar impacts delivered to the fibres. Calculated specific energy per bar impact, or refining intensity, has been shown to correlate well with pulp handsheet properties.
Optimizing refining conditions consists therefore in finding the appropriate combination of these two factors, namely specific energy and refining intensity. This problem is further complicated in a two-stage system where the operating conditions required to optimize pulp quality or energy efficiency in the second stage of refining will depend on the treatment applied to the fibres in the primary stage. Few studies have examined the interactions between refining stages over a significant range of conditions, and fewer still have attempted to quantify these effects.
Researchers at the Pulp and Paper Research Institute of Canada (PAPRICAN) have since developed a refining strategy based on pilot-scale pulping trials conducted over a specific range of conditions (see Tappi Journal, 1991, 74(3), 221-230; and Journal of Pulp and Paper Science, 1993, 19(1), J12-J18). The PAPRICAN strategy consists of applying the bulk of the specific energy used in two-stage refining at high intensity in the first stage of treatment. Subsequent commercial scale trials conducted at the Kruger Company's Bromptonville facility lead to three papers published in the patent literature assigned to Andritz Sprout-Bauer, namely U.S. Pat. No. 5,167,373, CA 2,094,674, and U.S. Pat. No. 5,248,099.
Andritz Sprout-Bauer (at one time called ABB Sprout-Bauer) has essentially patented the PAPRICAN two-stage refining strategy by designing and patenting machinery which physically embodies such treatment.
Although the good results obtained by PAPRICAN and Sprout-Bauer have provided evidence for the optimization of the energy efficiency and pulp quality produced by refining, there is still a great need to further improve this process.